Silicon production with a fluidized bed reactor integrated into a siemens-type process

ABSTRACT

A fluidized bed reactor and a Siemens reactor are used to produce polycrystalline silicon. The process includes feeding the vent gas from the Siemens reactor as a feed gas to the fluidized bed reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND

It is known that silicon can be made in rod form by a process referredto as the Siemens process. A mixture comprising hydrogen and silane(SiH₄) or a mixture comprising hydrogen and trichlorosilane is fed to adecomposition reactor containing, substrate rods which are kept at atemperature of more than 1000° C. Silicon is deposited on the substrateand by-product gas mixtures exit in a vent stream. When a mixturecomprising hydrogen and trichlorosilane is used, the vent stream mayinclude hydrogen, hydrogen chloride, chlorosilanes, silane, and siliconpowder. For purposes of this application, the term ‘chlorosilanes’refers to any silane species having one or more chlorine atoms bonded tosilicon and includes, but is not limited to monochlorosilane (H₃SiCl),dichlorosilane (H₂SiCl₂), trichlorosilane (HSiCl₃), silicontetrachloride (SiCl₄), and various chlorinated disilanes such ashexachlorodisilane and pentachlorodisilane. In the vent stream, hydrogenand chlorosilanes such as silicon tetrachloride and trichlorosilane maybe present both from un-reacted feed gas and reaction product from thedecomposition. The vent stream is passed through a complex recoveryprocess where condensations, scrubbing, absorption and adsorption areunit operations often used to facilitate the capture of feed materialtrichlorosilane and hydrogen for recycle. One problem associated withthe Siemens process is that it is difficult to achieve a high yield ofpolycrystalline silicon product to silicon fed due to the chemicalequilibria and kinetics that control this reaction process.

Quite often only 50%, or less, of the maximum theoretical yield ofpolycrystalline silicon is achieved.

An alternate process is to feed the mixture comprising hydrogen andsilane or the mixture comprising hydrogen and trichlorosilane to afluidized bed containing silicon nearly spherical beads that aremaintained also at high temperature. The beads grow in size, and whenlarge enough, are passed out the bottom of the fluidized bed reactor asproduct. The vent gases exit the top of the reactor and are sent througha recovery process similar to the one described above for the Siemensprocess. Yield in this system may be nearly 90% of theoretical maximum,as compared to the 50% for the Siemens process.

One problem with the fluidized bed reactor process is that one must heatthe beads to a temperature higher than the average bed temperature tofacilitate heat transfer. That can be done, for example, by use of hotwalled reactor, microwave energy, or infrared radiation. All heatingmethods have unique operating problems. One problem, however, is thatthe bottom of the fluidized bed reactor may be hot, and the feed gas isreactive when it contains only trichlorosilane and hydrogen. As aresult, the feed gas distributor, large beads, and reactor side wallsare prone to rapid deposition of silicon. Those deposits subsequentlydisrupt the proper feed distribution, product separation, and heattransfer of the system. Another problem with the fluidized bed reactorprocess is the product quality is generally insufficient for use inintegrated circuit manufacture; however, the product of the fluidizedbed reactor process may be used in solar grade applications.

There is a need in the polycrystalline silicon industry to improveefficiency of polycrystalline silicon production and reduce by-products.

SUMMARY

A process comprises feeding a vent gas stream from a Siemens reactor toa fluidized bed reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process described herein.

FIG. 2 is a flow diagram of an alternative process described herein.

REFERENCE NUMERALS

-   101 Siemens feed gas stream-   102 Siemens reactor-   103 polycrystalline silicon rod-   104 Siemens vent gas stream-   105 fluidized bed reactor-   106 supplement stream-   107 polycrystalline silicon bead product stream-   108 fluidized bed reactor vent gas stream-   109 recovery system-   110 hydrogen recovery line-   111 chlorosilane recovery line-   201 dust removing apparatus-   202 condensate line-   203 treated vent gas stream-   204 vaporizer-   205 overhead vapor line

DETAILED DESCRIPTION

To avoid duplicity of the feed and recovery systems for fluidized bedreactors and Siemens reactors, a fluidized bed reactor is placed suchthat the vent gas stream exiting a Siemens reactor is enters a fluidizedbed reactor as a feed gas stream. FIG. 1 shows a process flow diagram. ASiemens feed gas stream 101 is fed to a Siemens reactor 102 containing aU-rod. The Siemens feed gas stream may comprise trichlorosilane.Alternatively, the Siemens feed gas stream may comprise silane. TheU-rod may comprise two polycrystalline silicon seed rods connectedtogether by a polycrystalline silicon bridge. Polycrystalline silicon isdeposited from the feed gas stream 101 onto the U-rod to producepolycrystalline silicon product in rod form 103. The product in rod form103 is removed from the Siemens reactor 102 at the end of a batch. Thevent gas stream 104 from the Siemens reactor may comprisetrichlorosilane, silicon tetrachloride, hydrogen, hydrogen chloride andsilicon powder.

The vent gas stream 104 is fed into a fluidized bed reactor 105containing silicon seed particles. This vent gas stream 104 mayoptionally be supplemented with additional feed gases, with additionalinert gases, or both, in supplement stream 106. The supplement stream106 may comprise additional chlorosilanes. The additional chlorosilanesmay comprise trichlorosilane, silicon tetrachloride, or combinationsthereof. Polycrystalline silicon is deposited from the feed gasstream(s) 104, 106 onto the silicon seed particles. Polycrystallinesilicon product in bead form is removed from the fluidized bed reactor105 in product stream 107. A vent gas stream 108 may comprise hydrogen,hydrogen chloride, and chlorosilanes, e.g., trichlorosilane and silicontetrachloride, is removed from the fluidized bed reactor 105 and sent torecovery system 109. Hydrogen may be recovered and sent back to theSiemens reactor 102 through line 110. Chlorosilanes may be recoveredthrough line 111 and recycled or sold. Hydrogen chloride may berecovered and sold. Silicon tetrachloride may be hydrogenated orotherwise converted to trichlorosilane, and the resultingtrichlorosilane may be recycled to the Siemens reactor 102.

Siemens Reactor

The Siemens reactor used in this invention may be a conventional Siemensreactor, such as a Siemens reactor disclosed in U.S. Pat. Nos.2,999,735; 3,011,877; 3,862,020; or 3,961,003. For example, operation ofthe Siemens reactor may be performed as follows. Polycrystalline siliconseed rods are placed upright and parallel to one another in the Siemensreactor. Two or more of these seed rods may be connected to one anotherby a bridge, thereby forming a U-rod. The U-rods are heated until theyreach a temperature ranging from 700° C. to 1,400° C., alternatively1,000° C. to 1,200° C., alternatively 1,100° C. to 1,150° C. The Siemensreactor may be operated at a pressure ranging from 13 kPa (2 psig) to3450 kPa (500 psig), alternatively 6 kPa (1 psig) to 1380 kPa (200psig), and alternatively 100 kPa (1 bar) to 690 kPa (100 psig).

The Siemens feed gas is fed to the Siemens reactor through an inlet inthe base. The Siemens feed gas may comprise hydrogen andtrichlorosilane. The Siemens feed gas may optionally further comprisesilicon tetrachloride. Silicon is deposited from the feed gas onto theU-rod, thereby increasing the diameter of the U-rod. The Siemens feedstream may comprises 5% to 75% trichlorosilane. The Siemens feed gas maycomprise 0.015 moles of trichlorosilane per mole of hydrogen to 0.3moles of trichlorosilane per mole of hydrogen. Alternatively, theSiemens feed gas may comprise 0.03 moles of trichlorosilane per mole ofhydrogen to 0.15 moles of trichlorosilane per mole of hydrogen. Withoutwishing to be bound by theory, it is thought that the amount ofpolycrystalline silicon product ranging from 20% to 40% based on thetotal quantity of silicon contained in the Siemens feed gas may beobtained from the Siemens reactor.

The vent gas stream from the Siemens reactor may be fed directly to thefluidized bed reactor without intervening treatment steps (without anyunit operations between the Siemens reactor and the fluidized bedreactor). Alternatively, the vent gas stream from the Siemens reactormay be treated to remove certain species before being fed into thefluidized bed reactor. FIG. 2 shows a process flow diagram of thisprocess. The vent gas stream 104 from the Siemens reactor 102 may betreated, for example, by feeding the vent gas stream 104 through a dustremoving apparatus 201, which is cooled with fluid such as servicewater, thereby removing fine silicon powder, disilanes, or combinationsthereof through line 202. The dust removing apparatus 201 may comprise asintered metal blowback filter, a contact condenser, or a combinationthereof (for example, a sintered metal blowback filter located eitherupstream or downstream of a contact condenser in the stream 104 line.The resulting treated vent gas stream 203 comprising trichlorosilane(i.e., with fine silicon powder, disilanes, or both, removed) may thenbe heated, using for example, a vaporizer 204. The overhead vapor fromthe contact condenser 205 comprises hydrogen and non-condensablechlorosilanes. The overhead vapor 205 and the treated vent gas stream203 may optionally then be recombined and fed to the fluidized bedreactor 105. Without wishing to be bound by theory, it is thought thatalthough this treatment step requires more energy and capital thandirectly feeding the vent gas stream 104 from the Siemens reactor 102 tothe fluidized bed reactor 105, it is still more efficient than buildinga stand-alone process because existing capital assets from the Siemenstechnology can be used.

Fluidized Bed Reactor

The fluidized bed reactor used in this invention may be a conventionalfluidized bed reactor, such as a fluidized bed reactor disclosed in U.S.Pat. No. 5,077,028. For example, operation of the fluidized bed reactormay be performed as follows. Seed particles of silicon are placed in afluidized bed reactor and fluidized. Sources of seed particles are knownin the art. For example, seed particles may be obtained by mechanicalattrition of granular polycrystalline silicon or by crushingpolycrystalline silicon produced in a Siemens reactor. The gas used tofluidize the bed may comprise the vent gas stream from the Siemensreactor; a diluent gas such as hydrogen, argon, helium, nitrogen; or acombination thereof. Silicon is deposited on the surface of the seedparticles, increasing their diameters. The resulting product in beadform may be removed from the fluidized bed, and more seed particles maybe introduced.

The temperature inside the fluidized bed reactor may range from 900° C.to 1420° C., alternatively 1100° C. to 1300° C., and alternatively 1100°C. to 1250° C. Feeding the vent gas stream from the Siemens reactordirectly into the fluidized bed reactor may offer the advantage ofenergy savings by having to provide less heat to the fluidized bedreactor. The concentration of chlorosilanes in the feed stream to thefluidized bed reactor may range from 20 mol % to 50 mol %. Withoutwishing to be bound by theory, it is thought that excessive amounts offine powder may form if the concentration of chlorosilanes is higherthan 50%. The average diameter of the fluidized silicon particles mayrange from 0.5 mm to 4 mm, alternatively 0.6 mm to 1.6 mm. The residencetime of the fluidized bed may range from 0.5 second to 2 seconds. Theminimum fluidization velocity and design operational velocity may bedetermined by one of ordinary skill in the art based on various factors.The minimum fluidization velocity may be influenced by factors includinggravitational acceleration, fluid density, fluid viscosity, solidsdensity, and solids particle size. The operational velocity may beinfluenced by factors including heat transfer and kinetic properties,such as height of the fluidized bed, total surface area, flow rate ofsilicon precursor in the feed gas stream, pressure, gas and solidstemperature, concentrations of species, and thermodynamic equilibriumpoint.

One skilled in the art will recognize that the Siemens reactor operatesin a batch process, and the fluidized bed reactor operates in acontinuous process. Furthermore, the vent gas composition leaving theSiemens reactor may vary during the course of a batch. Therefore, oneskilled in the art would recognize that vent gases from multiple (two ormore) Siemens reactors may be combined to form a vent gas stream fed tothe fluidized bed reactor, or the feed gas stream to the fluidized bedreactor may be supplemented with additional trichlorosilane, silicontetrachloride, hydrogen, or a combination thereof, for example, tominimize variability of the feed gas stream to the fluidized bedreactor. Furthermore, the vent gas stream from the Siemens reactor maybe fed to one or more fluidized bed reactors configured in parallel.Without wishing to be bound by theory, it is thought that supplementingthe feed gas stream to the fluidized bed reactor with chlorosilanescomprising trichlorosilane may increase silicon production rate. Withoutwishing to be bound by theory, it is thought that supplementing the feedgas stream to the fluidized bed reactor with silicon tetrachloride mayprevent undesired deposition such as on heater walls and feeddistributors.

Without wishing to be bound by theory, the fluidized bed reactor mayhave deposition of the difference of yield, 90% to 50%, or 40% oftheoretical maximum. Without wishing to be bound by theory it is thoughtthat another advantage of this process is that the partially-convertedfeed gases from the Siemens reactor are of a composition that is notable to deposit silicon at temperatures above 1250° C. That detailallows for design of heating system by a hot wall reactor,resistively-heated feed tube, or other means more efficient thancommonly used in a fluidized bed reactor process.

The disclosure of ranges includes the range itself and also anythingsubsumed therein, as well as endpoints. For example, disclosure of arange of 700 to 1,400 includes not only the range of 700 to 1,400, butalso 700, 850, 1000 and 1400 individually, as well as any other numbersubsumed in the range. Furthermore, disclosure of a range of, forexample, 700 to 1400 includes the subranges of, for example, 1000 to1400 and 1000 to 1100, as well as any other subrange subsumed in therange. Similarly, the disclosure of Markush groups includes the entiregroup and also any individual members and subgroups subsumed therein.For example, disclosure of the Markush group hydrogen, trichlorosilane,tetrachlorosilane, and hydrogen chloride includes the member hydrogenindividually; the subgroup trichlorosilane and tetrachlorosilane; andany other individual member and subgroup subsumed therein.

Recovery System

The vent gas stream from the fluidized bed reactor may be recovered byany conventional means. The vent gas stream from the fluidized bedreactor may be cooled using conventional equipment. Fine silicon powdermay be removed using conventional equipment such as a contact condenser,sintered metal blowback filtration assembly, or a combination of acyclone and filter assembly.

Alternatively, the vent gas stream from the fluidized bed reactor may befed to a contact condenser to knock down the solids in liquidchlorosilanes and thereafter spray dry the fine silicon powder may bespray dried in a spray dryer. The resulting silicon powder may beneutralized and sold. Alternatively, the fine silicon powder andchlorosilanes may be recovered and converted to chlorosilanes for use asa feed stream to the Siemens reactor. One skilled in the art would beable to select a suitable recovery system without undue experimentation.

INDUSTRIAL APPLICABILITY

The combined benefits of no duplicity of feed and recovery systems andeasier heating of the process may make the integral fluidized bedreactor with a Siemens reactor process more manageable and economic. Thepolycrystalline silicon product of the Siemens reactor may be suitablefor either solar cell or integrated circuit applications. Thepolycrystalline silicon product of the fluidized bed reactor may besuitable for solar cell applications.

1. A process comprising: feeding a Siemens feed gas stream comprisingsilane or trichlorosilane in an amount of from 5 to 75 mol % to one ormore Siemens reactors containing polycrystalline silicon seed rods;producing polycrystalline silicon in the one or more Siemens reactorsand forming a vent gas stream having 20 to 50 mol % chlorosilanes;directly feeding the vent gas stream from the one or more Siemensreactors without intervening treatment steps to one or more fluidizedbed reactors containing seed particles of silicon, where the temperatureof the one or more fluidized bed reactors ranges from 900 to 1420° C.,where residence time in the fluidized bed ranges from 0.5 to 2 seconds,and where polycrystalline silicon is produced in the one or morefluidized bed reactors.
 2. The process of claim 1, where the vent gasstream comprises hydrogen and chlorosilanes.
 3. The process of claim 1,where the vent gas stream comprises hydrogen and silane.
 4. The processof claim 2, further comprising supplementing the vent gas stream withadditional chlorosilanes.
 5. The process of claim 4, where theadditional chlorosilanes comprise trichlorosilane, silicontetrachloride, or a combination thereof.
 6. The process of claim 2,where the vent gas stream comprises trichlorosilane, silicontetrachloride, hydrogen, hydrogen chloride and silicon powder.
 7. Theprocess of claim 2, where the vent gas stream is supplemented withadditional trichlorosilane to form a feed gas stream to the fluidizedbed reactor.
 8. The process of claim 2, further comprising: 2) feeding asecond vent gas stream from the fluidized bed reactor to a recoverysystem.
 9. The process of claim 8, where the second vent gas streamcomprises: hydrogen, trichlorosilane, tetrachlorosilane, and hydrogenchloride.
 10. The process of claim 8, further comprising: 3) recoveringhydrogen, trichlorosilane, or both, and feeding the hydrogen,trichlorosilane, or both, to the Siemens reactor in step 1).
 11. Theprocess of claim 8, further comprising 3) recovering tetrachlorosilane,converting the tetrachlorosilane to trichlorosilane, and feeding thetrichlorosilane to the Siemens reactor in step 1).
 12. A processcomprising: feeding a Siemens feed gas stream comprising silane ortrichlorosilane in an amount of from 5 to 75 mol % to multiple Siemensreactors containing polycrystalline silicon seed rods; producingpolycrystalline silicon in the multiple Siemens reactors; combining ventgases from the multiple Siemens reactors to form a vent gas streamhaving a concentration of from 20 to 50 mol % of chlorosilanes, andfeeding the vent gas stream directly from the multiple Siemens reactorswithout intervening treatment steps to one or more fluidized bedreactors containing seed particles of silicon, where the temperature ofthe one or more fluidized bed reactors ranges from 900 to 1420° C.,where residence time in the fluidized bed reactors ranges from 0.5 to 2seconds, and where polycrystalline silicon is produced in the one ormore fluidized bed reactors.
 13. The process of claim 12, where the ventgas stream comprises hydrogen and chlorosilanes.
 14. The process ofclaim 12, where the vent gas stream comprises hydrogen and silane. 15.The process of claim 13, where the vent gas stream comprisestrichlorosilane, silicon tetrachloride, hydrogen, hydrogen chloride andsilicon powder.
 16. The process of claim 13, further comprising: ii)feeding a second vent gas stream from the fluidized bed reactor to arecovery system.
 17. The process of claim 16, where the second vent gasstream comprises: hydrogen, trichlorosilane, tetrachlorosilane, andhydrogen chloride.
 18. The process of claim 16, further comprising: iii)recovering hydrogen, trichlorosilane, or both, and feeding the hydrogen,trichlorosilane, or both, to the Siemens reactor in step i).
 19. Theprocess of claim 16, further comprising iii) recoveringtetrachlorosilane, converting the tetrachlorosilane to trichlorosilane,and feeding the trichlorosilane to the Siemens reactor in step i).
 20. Aprocess comprising: feeding a Siemens feed gas stream comprising silaneor trichlorosilane to one or more Siemens reactors containingpolycrystalline silicon seed rods; producing polycrystalline silicon inthe one or more Siemens reactors and forming a vent gas stream having 20to 50 mol % chlorosilanes; directly feeding the vent gas stream from theone or more Siemens reactors without intervening treatment steps to oneor more fluidized bed reactors containing seed particles of siliconduring production of polycrystalline silicon, where the vent gas streamcomprises tetrachlorosilane in addition to silane or trichlorosilane,where residence time in the fluidized bed ranges from 0.5 to 2 seconds,and where polycrystalline silicon is produced in the one or morefluidized bed reactors.
 21. The process of claim 20 where seed particlesof silicon are placed in the fluidized bed reactor and fluidized. 22.The process of claim 20 where the vent gas stream from the one or moreSiemens reactors is of a composition that is not able to deposit siliconat temperatures above 1250° C.
 23. The process of claim 20 where thevent gas stream comprises hydrogen and chlorosilanes, and where theprocess further comprises the step of ii) feeding a second vent gasstream from the one or more fluidized bed reactors to a recovery system.24. A process comprising: feeding a Siemens feed gas stream comprisingsilane or trichlorosilane to multiple Siemens reactors containingpolycrystalline silicon seed rods; combining vent gases from themultiple Siemens reactors to form a vent gas stream having 20 to 50 mol% chlorosilanes, and directly feeding the vent gas stream from themultiple Siemens reactors without intervening treatment steps tofluidized bed reactors configured in parallel and containing seedparticles of silicon, where residence time in the fluidized bed reactorsranges from 0.5 to 2 seconds, where the vent gas stream comprisestetrachlorosilane in addition to silane or trichlorosilane, wherepolycrystalline silicon is produced in the one or more fluidized bedreactors, and where polycrystalline silicon is produced in the Siemensreactors.
 25. The process of claim 24, where the vent gas streamcomprises hydrogen and silane.
 26. The process of claim 24, where thevent gas stream comprises hydrogen and chlorosilanes.
 27. The process ofclaim 26, further comprising supplementing the vent gas stream withadditional chlorosilanes.
 28. The process of claim 27, where theadditional chlorosilanes comprise trichlorosilane, silicontetrachloride, or a combination thereof.
 29. The process of claim 26,where the vent gas stream comprises trichlorosilane, silicontetrachloride, hydrogen, hydrogen chloride and silicon powder and theprocess further comprises removing silicon powder from the vent gasstream before feeding the vent gas stream to the fluidized bed reactor.30. The process of claim 26, where the vent gas stream is supplementedwith additional trichlorosilane to form a feed gas stream to thefluidized bed reactor.
 31. The process of claim 26, further comprising:2) feeding a second vent gas stream from the fluidized bed reactor to arecovery system.
 32. The process of claim 31, where the second vent gasstream comprises: hydrogen, trichlorosilane, tetrachlorosilane, andhydrogen chloride.
 33. The process of claim 31, further comprising: 3)recovering hydrogen, trichlorosilane, or both, and feeding the hydrogen,trichlorosilane, or both, to the Siemens reactor in step 1).
 34. Theprocess of claim 31, further comprising 3) recovering tetrachlorosilane,converting the tetrachlorosilane to trichlorosilane, and feeding thetrichlorosilane to the Siemens reactor in step 1).